1. Technical Field
This invention relates to methods and apparatus for the installation of telecommunications cables, in particular optical fibre installed into pre-installed optical fibre tubes by “blowing” techniques.
2. Description of Related Art
The method and apparatus used to install optical fibre transmission lines into optical fibre tubes or ducts using the viscous drag provided a high-speed flow of a fluid medium, often air, is known from EP108590 and subsequent publications. A blowing head is used for the installation of the optical fibre unit into the optical fibre tubes or ducts. (In this description, references to “fibre” and “fibre units” shall be deemed to include individual fibre members and fibre bundles, and vice versa, as the context allows.)
The blowing head comprises a chamber, into which pressurised air is pumped. The air is directed to flow into the mouth of a fibre tube, and then through the tube which is connected to the blowing head. The fibre unit is initially fed into the tube by a pushing force, so that when there is sufficient fibre surface within the tube for pressurised air to work on, the effects of viscous drag take over at least part of the task of advancing the fibre within the tube.
In use, the blowing heads of the prior art suffer from a number of problems.
First, it was found that the fibre unit was susceptible to buckling during installation. As discussed in EP253636, optical fibre is flexible and necessarily smaller in cross section than the fibre tube it is populating. For example, part of the advancing fibre unit could stop moving within the tube due to excessive friction build up between the fibre and the interior of the tube. A buckle develops if the blowing head continues to drive the fibre unit regardless. A buckled fibre unit could adversely affect the performance of the fibre when installed, or even physically damage it. At the least, buckling would delay the installation process.
This problem of fibre buckle was addressed in EP253636 and WO98/12588, wherein methods and apparatus are described to sense fibre buckle, feed back the existence of a buckle in the fibre unit to the blowing head, and then to use the information to adjust the pushing force driving the fibre forward. In these solutions, the effect of fibre buckle within the tube is “transmitted” back to the blowing head and the fibre unit will buckle into a “buckle cavity”. Sensors are located within the cavity to detect the buckle.
JP H04-335604 similarly proposes a method in a magnetic clutch-based blown fibre unit installation system, to use sensed information to control the pushing force applied. The sensing is done not by detecting buckles, but by sensing with an ammeter, the load put on the pushing mechanism during an installation. The aim is to provide smooth and controlled playout of fibre by the blowing head, and thus to avoid buckle. However this method is unlikely to achieve that end as the method and apparatus proposed is not sufficiently responsive nor repeatable owing to a hysteresis loop lag in a magnetic clutch system.
A second problem concerned the amount of air leakage from the blowing head. Air is fed into the chamber of the blowing head under considerable pressure, typically from 5 to 15 bar. This high pressure is required because a fibre tube has a very small internal diameter (typically not exceeding 3.5 mm by today's standards), but may be of very great length: fibre tubes populated by the blowing technique which exceed 1,000 meters are currently not uncommon. The chamber of the blowing head, being comprised of a bore, is typically about 1.1 to 1.2 mm. Air fed into the chamber will seek escape at high pressure from every possible vent and fissure in the blowing head.
Also, not all blowing sessions involved the fibre unit being fed into the mouth of a tube, and to have the fibre emerge at the other end of the tube. Sometimes a blowing session would start from an intermediate point in the intended path of the fibre when installed; this is sometimes known as a bi-directional installation. Such an installation method can be used to populate longer tubes, where the total distance to be covered exceeds that possible in a single blowing session. In brief, one tip of the fibre unit is fed into a first tube and blown in one direction until the end emerges from the far end of the tube; the process is repeated using the other tip of the fibre unit and blowing it in the opposite direction. To cope with the change of blowing direction in bi-directional installations, WO98/12588 shows how the blowing head can be opened along the line of the fibre unit travel, allowing the user to remove the installed fibre after completing the first part of the task. This however means that the blowing head is now made up of typically two halves which have to be sealed shut (e.g. by clamping) during an installation session. There are thus numerous points of escape for the pressurised air: not only at the two ends of the bore making up the chamber (where the fibre unit enters and exits), but also along the seams where the parts of the blowing head meet when clamped shut.
Deformable seals were typically used to defend against air leakage, but these proved to lack durability on account of its exposure to the glass microspheres which coat the protective sheath of a fibre unit or bundle. The glass beads are used on blown fibre units to reduce the friction generated between the fibre and the inside tube surface, as further described in e.g. EP186753. As deformable seals are typically made from rubber or such materials, they are highly susceptible to damage by the glass, making frequent replacement a costly necessity.
As a result of air leakage from the blowing head, the amount available to generate the required viscous drag within the fibre tube decreases. It is thus necessary to employ expensive high-volume air compressors to compensate for the loss of air. In addition to the expense of procuring and operating such compressors to make good the wastage, the weight and bulk of the machinery has necessitated the employment of more than one operative, with associated cost implications.
A third problem arises from developments in the size of fibre bundles (comprising a number of fibre units or members) and the size of fibre tubes. British Telecommunications plc in the UK deploys, or has deployed in the past 18 years, bundles ranging from 2 to 12 fibres members. Tube sizes vary accordingly. It is unknown what other sizes may be adopted in the future. While the blowing heads of the prior art attempt to build in a measure of flexibility in the range of fibre bundles and tubes they can handle, the sheer range in sizes in current use means that a single blowing head capable of handling the entire range of sizes would be cost-efficient and greatly advantageous.
Yet another problem with blowing heads of the prior art has been cost: cost in terms of manufacture and in operation. Up to now, the experience has been that blown fibre has been deployed chiefly in the business or commercial context. This is because the need for fibre-based communications outweighs the cost of obtaining it. For residential users however, “last mile” issues—where ultimate users still use limited bandwidth copper wire in an otherwise all-fibre network—arise in no small part to the cost-sensitivity of such customers.
As a result of lower take up in the residential sphere, there is no real critical mass for the deployment of blowing head. However, it is anticipated that with rising consumer demand, ubiquitous fibre to the home (“FTTH”) will become a reality in the United Kingdom and elsewhere in the near future. The provision of a low-cost fibre installation service at high volumes becomes crucial to the provision of this service. Indeed, cost is a major factor in determining the rate of adoption of FTTH.
There is thus a need for a blowing head that can be manufactured at a low price, and which can be operated cheaply. As cost used to be less of a consideration, the blowing heads of the prior art tended to be seen as specialised pieces of equipment tooled from expensive materials. Owing to high levels of air leakage in use, powerful and expensive air compressors had to be used with the prior art blowing heads.
Prior art blowing heads had also to be operated by skilled users. Each blowing session is unique. For example, there are differences in the size of fibre/fibre bundle and tube, length of tube to be populated, the atmospheric conditions (e.g. dewpoint levels affect the quality of the air pumped into the blowing head). The users need to be able to accurately read the conditions to ensure the correct setup of the blowing head. Moreover they need to be alert to the possibility of problems such as fibre buckle, and to take quick remedial steps by making adjustments to the blowing head. Aside from the need for skilled operators, prior art blowing heads required at least two people in an installation session, which was due in part to the need for a large compressor needing more than one person to move and to set up.
In short, prior art blowing heads are too expensive to make and to use, to be sensibly feasible for mass deployment to provide fibre connections to private premises.